Docker: Remove custom pip install for Alpine docker (#2028)

This PR simply removes the installation of pip from the Alpine Docker image. In the past, this installation was necessary because there was an issue with pip 20.0.0, so another version was required. This is no longer needed as the Alpine version we use delivers pip 21.3.1.

Fixes #2027.

docker/alpine/Dockerfile

@@ -189,8 +189,6 @@ ENV LC_ALL="en_US.UTF-8"
 # Copy GRASS GIS from build image
 COPY --from=build /usr/local/bin/grass /usr/local/bin/grass
 COPY --from=build /usr/local/grass* /usr/local/grass/
-# pip 20.0.0 fix
-RUN apk add curl && curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py && python3 get-pip.py pip==20.0.2 && rm get-pip.py
 
 # install external Python API
 RUN pip3 install --upgrade pip six grass-session --ignore-installed six